Conventionally, a capacitive humidity sensor having a humidity sensitive film is disclosed in, for example, U.S. Pat. No. 6,580,600. The relative permittivity of the humidity sensitive film is changeable in accordance with humidity. The humidity sensitive film is disposed between a pair of electrodes.
In the sensor, the electrodes are disposed on the same plan of a substrate, and face each other. The humidity sensitive film is disposed between the electrodes and covers the electrodes.
In a manufacturing method of the above sensor, a paste including high polymer material is printed on the substrate by a screen printing method, and then, the paste is hardened so that the humidity sensitive film is formed. In this case, a step of patterning the humidity sensitive film is not necessary, compared with a spin coating method for coating the humidity sensitive film on the substrate. Thus, the manufacturing method including the screen printing step is simplified.
In the above sensor, a pair of electrodes provides a capacitor including the humidity sensitive film as a dielectric layer between the electrodes. On the basis of the capacitance change of the capacitor, the humidity sensor detects the humidity. In this case, the capacitance is affected by not only the humidity sensitive film between the electrodes but also the humidity sensitive film on the electrodes. The capacitance derived from the humidity sensitive film on the electrodes is a fringing capacitance. Further, an initial capacitance of the above sensor is comparatively small, compared with a parallel plate type sensor. Thus, the sensitivity of the above sensor is much affected by thickness deviation of the humidity sensitive film.
Here, when the humidity sensitive film is formed by the screen printing method, the thickness of the humidity sensitive film depends on a physical condition of the paste such as viscosity, a condition of the surface of the substrate such as wettability of the paste, a screen printing condition such as dimensions of an opening of the pattern, i.e., an opening degree of a screen, a thickness of emulsion and a screen tension, and a squeegee condition such as a scanning speed and a printing pressure. Specifically, the thickness of a part of the humidity sensitivity film around a periphery of the opening of the pattern and near a screen mesh is much deviated from a desired thickness.